Cd(II) and Zn(II) thiocyanate coordination compounds with 3-ethylpyridine: Synthesis, crystal structures and properties

Article abstract of DOI:10.1515/znb-2017-0186

Reaction of Cd(NCS)₂ and Zn(NCS)₂ with 3-ethylpyridine leads to the formation of compounds of compositions M(NCS)₂(3-ethylpyridine)₄ (M=Cd, 1-Cd; Zn, 1-Zn) and M(NCS)₂(3-ethylpyridine)₂ (M=Cd, 2-Cd; Zn, 2-Zn). 1-Cd and 1-Zn are isotypic and form discrete complexes in which the metal cations are octahedrally coordinated by two trans-coordinating N-bonded thiocyanate anions and four 3-ethylpyridine co-ligands. In 2-Cd the cations are also octahedrally coordinated but linked into chains by pairs of μ-1,3-bridging anionic ligands. 2-Zn is built up of discrete complexes, in which the Zn cation is tetrahedrally coordinated by two N-bonded thiocyanate anions and two 3-ethylpyridine co-ligands. Compounds 1-Cd, 2-Cd and 2-Zn can be prepared in a pure state, whereas 1-Zn is unstable and transforms on storage into 2-Zn. If 1-Cd and 1-Zn are heated, a transformation into 2-Cd, respectively 2-Zn is observed. Luminescence measurements reveal that 1-Cd, 2-Cd and 2-Zn emit light in the blue spectral range with maxima at, respectively, 21724, 21654 and 22055 cm^-1, assigned to ligand-based luminescence.

Full text of DOI:10.1515/znb-2017-0186

Z. Naturforsch. 2018; 73(2)b: 115–123
Tristan Neumann, Inke Jess, Cesar dos Santos Cunha, Huayna Terraschke
and Christian Näther*
Cd(II) and Zn(II) thiocyanate coordination
compounds with 3-ethylpyridine: synthesis,
crystal structures and properties
https://doi.org/10.1515/znb-2017-0186
important field in coordination chemistry. One of the
Received November 15, 2017; accepted November 29, 2017
major advantages of this class of compounds is the fact,
that based on simple considerations concerning the coor-
dination properties of metal cations, anionic ligands and
neutral co-ligands, the dimensionality of their coordina-
tion networks can be predicted to some extent [1–6]. In
this context numerous compounds have been reported
that show different physical properties like, e.g. gas sorp-
tion, luminescence or magnetism [7–16].
Abstract: Reaction of Cd(NCS) and Zn(NCS) with 3-ethyl-
2
2
pyridine leads to the formation of compounds of compo-
sitions M(NCS) (3-ethylpyridine) (Mꢀ=ꢀCd, 1-Cd; Zn, 1-Zn)
2
4
and M(NCS) (3-ethylpyridine) (Mꢀ=ꢀCd, 2-Cd; Zn, 2-Zn).
2
2
1-Cd and 1-Zn are isotypic and form discrete complexes in
which the metal cations are octahedrally coordinated by
two trans-coordinating N-bonded thiocyanate anions and
four 3-ethylpyridine co-ligands. In 2-Cd the cations are
also octahedrally coordinated but linked into chains by
pairs of μ-1,3-bridging anionic ligands. 2-Zn is built up of
discrete complexes, in which the Zn cation is tetrahedrally
coordinated by two N-bonded thiocyanate anions and
two 3-ethylpyridine co-ligands. Compounds 1-Cd, 2-Cd
and 2-Zn can be prepared in a pure state, whereas 1-Zn is
unstable and transforms on storage into 2-Zn. If 1-Cd and
Our interest focuses on coordination compounds and
polymers based on 3d transition metal cations and thiocy-
anate ligands with additional N-donor co-ligands [17–24].
This anionic ligand shows a large variety of coordination
modes with the N-terminal and the μ-1,3-bridging mode
as the most prominent [25–32]. Compounds with a bridg-
ing coordination are of special interest, because mag-
netic exchange can be mediated [33–39]. Unfortunately,
the metal cations in which we are interested like Mn(II),
Fe(II), Co(II) and Ni(II) are not very chalcophilic and
usually prefer a terminal N-coordination of the anionic
ligands and therefore, the synthesis of the desired com-
pounds with a bridging coordination is sometimes dif-
ficult to perform. In this case, coordination compounds
1
-Zn are heated, a transformation into 2-Cd, respectively
2
-Zn is observed. Luminescence measurements reveal that
1
-Cd, 2-Cd and 2-Zn emit light in the blue spectral range
−1
with maxima at, respectively, 21724, 21654 and 22055 cm ,
assigned to ligand-based luminescence.
Keywords: 3-ethylpyridine; Cd and Zn coordination com- with terminal thiocyanate anions can be heated, which in
pounds; crystal structures; luminescence properties; most cases leads to a stepwise removal of the co-ligands
thermal properties.
and the formation of the desired compounds in which
the metal cations are linked by μ-1,3-bridging anionic
ligands [40, 41]. Following this procedure microcrystal-
line powders are obtained, that cannot be characterized
by single crystal X-ray diffraction. To obtain structural
information, similar compounds with Cd(II) can be pre-
1
Introduction
Investigations on the synthesis, crystal structures and pared, since Cd(II) is much more chalcophilic and there-
properties of new coordination polymers is still an fore the bridging compounds are usually more stable
than those with a terminal coordination of the anionic
ligands. In many cases they are isotypic to the paramag-
*
Corresponding author: Christian Näther, Institut für Anorganische
netic analogs and thus, the structure of the latter can be
determined by Rietveld refinements [42]. It is noted, that
thermal decomposition of Co(II) compounds sometimes
lead to the formation of tetrahedral complexes, which
exhibit the same ratio between the metal thiocyanate
and the organic co-ligand. Such intermediates can be
Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Straße 2,
2
4118 Kiel, Germany, Fax: +49-431-8801520,
E-mail: cnaether@ac.uni-kiel.de
Tristan Neumann, Inke Jess, Cesar dos Santos Cunha and Huayna
Terraschke: Institut für Anorganische Chemie, Christian-Albrechts-
Universität zu Kiel, Max-Eyth-Straße 2, 24118 Kiel, Germany
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1
16ꢀꢀꢀꢀꢁꢀT. Neumann et al.: New Cd(II) and Zn(II) thiocyanate coordination compounds
identified by the preparation of corresponding Zn com- compounds (2-Cd and 2-Zn) are not isotypic and XRPD
pounds which are usually tetrahedral coordinated [43]. measurements indicate that most batches of 1-Zn are con-
This is the main reason why we are generally interested taminated with 2-Zn. For all compounds suitable single
in the chemical and structural properties of coordination crystals were grown and investigated by X-ray diffraction.
compounds based on Cd(II) and Zn(II) thiocyanate. In
this context, it is noted that these compounds frequently
show luminescence, and several of them are reported 2.2 Crystal structures
in literature [44–49]. Even if this luminescence is based
predominantly on the organic ligands, we became inter- Compounds 1-Cd and 1-Zn are isotypic and crystallize in
ested in this behavior to check if differences are observed the triclinic space group P
̅
1 with one formula unit in the
between discrete compounds or coordination polymers or unit cell. The asymmetric unit consists of one metal cation
between compounds with different metal cations or ter- which is located on a center of inversion, and one thiocy-
minal or bridging ligands [50–52].
anate anion as well as two 3-ethylpyridine ligands that are
In the course of our systematic investigations we located in general positions (Fig. 1).
predominantly used pyridine derivatives that are substi-
The crystal structures consist of discrete complexes
tuted at the 4-position, that, except of Zn(II), mostly form in which the cations are octahedrally coordinated by four
coordination polymers with bridging anionic ligands. 3-ethylpyridine co-ligands and two terminal N-bonded
We also prepared some compounds where the substitu- thiocyanate anions (Fig. 1). For 1-Cd the Cd–N distances
ent is directly neighbored to the coordinating N atom. are between 2.2990(2) and 2.389(2) Å which is in the usual
In this case the synthesis of coordination polymers is range observed for such compounds. As expected, in
more difficult and with, e.g. Co(II) discrete complexes 1-Zn the Zn–N distances are shortened to values between
were obtained, presumably because of steric reasons 2.106(2) and 2.235(1) Å. In both compounds all angles
[
43]. Therefore we used e.g. 3-hydroxymethyl or 3-amino- deviate from the ideal values, which shows that the octa-
methylpyridine in previous works, but in most cases com- hedra are slightly distorted.
pounds were obtained, in which the metal cations are
Compound 2-Cd also crystallizes in the triclinic space
linked via the pyridine co-ligand, which prevented the group P
̅
1 and the asymmetric unit consists of one Cd
formation of metal thiocyanate chains. To avert such a cation, two crystallographically independent thiocyanate
behavior we became interested in 3-ethylpyridine as a co- anions and two 3-ethylpyridine ligands, all of them being
ligand, for which no thiocyanate coordination polymers located in general positions (Fig. 2: top). One ethyl group
were reported. There are only two polymorphic modifica- of one co-ligand and a large part of the second co-ligand
tions of Ni(NCS) (3-ethylpyridine) reported in literature are disordered and were refined using a split model (see
2
2
that form discrete complexes, but compounds based on Experimental section). The Cd cations are coordinated
Cd(II) and Zn(II) are unknown [53].
by two 3-ethylpyridine ligands as well as two N and two
S-bonding thiocyanate anions within slightly distorted
octahedra (Fig. 2: top). The Cd–N distances ranges from
2
2
.305(3) to 2.341(3) Å and the Cd–S distances amount to
.7446(2) and 2.752(2) Å.
2
Results and discussion
The Cd cations are linked by pairs of anionic ligands
2.1 Synthesis
into linear chains in which the thiocyanate N and S atoms
as well as the 3-ethylpyridine N atoms are always in trans-
Preliminary synthetic investigations using different arrangement. This is a very common motif in this family
amounts of Cd(NCS) or Zn(NCS) and 3-ethylpyridine indi- of compounds (Fig. 2: bottom). Within these chains the
2
2
cated that only four crystalline phases are accessible from 6-membered rings of the co-ligands are alternatingly
solution. If an excess of the co-ligand is used, one Cd and rotated out of the chain vector, which can be traced back
one Zn compound are obtained, that might be isotypic. to steric repulsion between the co-ligands (Fig. 2: bottom).
Elemental analysis indicated a composition that is close to The intrachain Cd–Cd distance amounts to 5.875 Å and the
M(NCS) (3-ethylpyridine) (Mꢀ=ꢀCd; 1-Cd, Zn; 1-Zn). If the shortest interchain Cd–Cd distance is 9.755 Å.
2
4
reactants are used in a ratio of 1:2, an additional crystal-
Compound 2-Zn crystallizes in the monoclinic space
line phase is obtained for each cation in which the ratio group P2 /m with the Zn cations located on a crystallo-
1
between the metal salt and 3-ethylpyridine is 1:2. The two graphic mirror plane. The lattice parameters are close
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T. Neumann et al.: New Cd(II) and Zn(II) thiocyanate coordination compoundsꢂꢁꢀꢀꢀꢂ117
Fig. 2:ꢀCrystal structure of 2-Cd with labeling and displacement
ellipsoids drawn at the 50% probability level. Disordering of the
3
-ethylpyridine ligand is shown as full and open bonds (top), and
view of a single chain is presented (bottom). Symmetry codes:
Aꢁ=ꢁ–x, –yꢁ+ꢁ1, –zꢁ+ꢁ1. In the bottom figure the disorder is not shown.
to that for orthorhombic but the crystal symmetry is
clearly 2/m. Consequently, the crystals are twinned and
therefore, a twin-refinement was performed (see Experi-
mental section). Each of the Zn cations is tetrahedral
coordinated by two terminal N-bonded thiocyanate
anions and two 3-ethylpyridine ligands (Fig. 3). The
Zn–N distances to the anionic ligands amount to 1.936(5)
Fig. 1:ꢀMolecular structures of 1-Cd (top) and 1-Zn (bottom) with
labeling and displacement ellipsoids drawn at 50% probability
level. Symmetry codes: Aꢁ=ꢁ–xꢁ+ꢁ1, –yꢁ+ꢁ1, –zꢁ+ꢁ1.
Fig. 3:ꢀMolecular structure of 2-Zn with labeling and displacement
ellipsoids drawn at the 50% probability level. Symmetry codes:
Aꢁ=ꢁ–x, –yꢁ+ꢁ1, –zꢁ+ꢁ1.
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18ꢀꢀꢀꢀꢁꢀT. Neumann et al.: New Cd(II) and Zn(II) thiocyanate coordination compounds
and 1.949(5) Å and those to the neutral co-ligands are
elongated to 2.027(3) Å.
Based on the single crystal data, X-ray powder dif-
fraction (XRPD) patterns were calculated and compared
with the experimental patterns, which show that 2-Cd
and 2-Zn were obtained as pure phases, whereas 1-Zn
is always contaminated with small amounts of 2-Zn
(
Fig. 4).
The reason why 1-Zn is always contaminated with
-Zn can be traced back to the fact that 1-Zn is unsta-
ble and transforms rapidly into the tetrahedral complex
-Zn, which obviously is more stable. The transforma-
tion is complete within several days as proven by XRPD
Fig. 5).
2
2
(
Fig. 5:ꢀExperimental XRPD pattern of freshly prepared 2-Zn and after
storage for 10 days as well as calculated XRPD pattern of 1-Zn and 2-Zn.
2
.3 IR and Raman spectroscopic
investigations
All compounds were investigated by IR and Raman
spectroscopy to determine the value of the CN stretching
vibration, which might be indicative for the coordination
mode of the thiocyanate anion. For compound 1-Cd this
vibration is observed in the IR and in the Raman spectrum
−1
at 2054 cm , which is in perfect agreement with values for
terminally N-bonded thiocyanate anions retrieved from
literature (Table 1) [27, 54].
−1
In contrast, in 2-Cd this band is shifted to 2086 cm ,
−1
which is below the threshold of about 2100 cm expected
for μ-1,3-bridging thiocyanate ligands [27]. For 1-Zn the
−1
CN stretch is observed at 2068 and 2067 cm , which
is comparable to the values in 1-Cd and in agreement
with the fact that the CN stretching vibration for Zn is
usually at higher values compared to Cd [27]. In agree-
ment with the presence of terminal anionic ligands, for
−1
2
-Zn the CN stretch is observed at 2067 and 2070 cm
(
Table 2).
−1
Table 1:ꢀValues of the CN stretching vibration (cm ) in the IR and
Raman spectra of 1-Cd, 2-Cd, 1-Zn and 2-Zn.
Compound
-Cd
IR
Raman
Fig. 4:ꢀExperimental and calculated XRPD pattern of the Cd (top)
and the Zn compounds (bottom). It is noted, that the powder pattern
for 1-Zn was calculated using cell parameters retrieved from a
Pawley Fit of a XRPD pattern measured at room temperature and that
ꢁ
ꢃꢄꢅꢆ
ꢃꢄꢇꢈ
ꢃꢄꢈꢇ
ꢃꢄꢈꢉ
ꢃꢄꢅꢆ
ꢃꢄꢇꢈ
ꢃꢄꢈꢉ
ꢃꢄꢉꢄ
ꢂ-Cd
-Zn
ꢂ-Zn
ꢁ
1
-Zn is contaminated with a very small amount of 2-Zn.
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T. Neumann et al.: New Cd(II) and Zn(II) thiocyanate coordination compoundsꢂꢁꢀꢀꢀꢂ119
Table 2:ꢀSelected crystal data and details on the structure determinations for 1-Cd, 1-Zn, 2-Cd and 2-Zn.
ꢁ-Cd
ꢁ-Zn
ꢂ-Cd
ꢂ-Zn
Formula
C H N S Cd
C H N S Zn
C H N S Cd
C H N S Zn
ꢊꢄ ꢊꢈ
ꢈꢅꢉ.ꢋꢉ
ꢈ
ꢃ
ꢊꢄ ꢃꢇ
ꢈꢄꢃ.ꢄꢉ
ꢈ
ꢃ
ꢋꢈ ꢋꢇ
ꢆꢆꢃ.ꢇꢈ
ꢆ
ꢃ
ꢋꢈ ꢋꢇ
ꢊꢌꢅ.ꢇꢊ
ꢆ
ꢃ
−
ꢋ
Mw, g mol
Crystal system
Space group
a, Å
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Åꢊ
triclinic
triclinic
triclinic
monoclinic
P
̅
P
̅
P
̅
Pꢃ /m
ꢋ
ꢇ.ꢌꢅꢊꢄ(ꢉ)
ꢌ.ꢃꢄꢊꢃ(ꢇ)
ꢋꢄ.ꢊꢊꢈꢅ(ꢋꢋ)
ꢇꢉ.ꢉꢇꢆ(ꢋꢃ)
ꢇꢊ.ꢇꢌꢅ(ꢋꢋ)
ꢉꢋ.ꢋꢅꢊ(ꢋꢄ)
ꢇꢄꢋ.ꢆꢅ(ꢋꢆ)
ꢃꢄꢄ(ꢃ)
ꢇ.ꢉꢌꢌꢅ(ꢊ)
ꢌ.ꢋꢉꢅꢆ(ꢆ)
ꢋꢄ.ꢋꢌꢋꢇ(ꢆ)
ꢇꢇ.ꢇꢄꢆ(ꢊ)
ꢇꢊ.ꢆꢇꢌ(ꢊ)
ꢉꢋ.ꢊꢄꢅ(ꢊ)
ꢉꢉꢆ.ꢊꢊ(ꢅ)
ꢋꢉꢄ(ꢃ)
ꢌ.ꢉꢅꢆꢌ(ꢉ)
ꢋꢄ.ꢆꢌꢈꢈ(ꢌ)
ꢋꢋ.ꢊꢅꢈꢄ(ꢇ)
ꢌꢈ.ꢉꢋꢃ(ꢌ)
ꢋꢋꢆ.ꢉꢈꢊ(ꢇ)
ꢋꢋꢄ.ꢊꢈꢆ(ꢌ)
ꢌꢆꢋ.ꢅꢇ(ꢋꢅ)
ꢃꢄꢄ(ꢃ)
ꢅ.ꢊꢃꢌꢋ(ꢊ)
ꢋꢃ.ꢊꢃꢆꢉ(ꢌ)
ꢋꢆ.ꢋꢇꢊꢅ(ꢌ)
ꢌꢄ
ꢌꢄ.ꢄꢃꢇ(ꢉ)
ꢌꢄ
ꢌꢊꢋ.ꢅꢉ(ꢋꢄ)
ꢃꢄꢄ(ꢃ)
ꢃ
ꢋ.ꢆꢋ
ꢋ.ꢅꢅ
ꢃꢅ.ꢌꢌꢇ
ꢈꢄꢋꢇ
ꢋꢌꢄꢅ
T, K
Z
ꢋ
ꢋ
ꢃ
−
ꢊ
Dcalcd, g cm
ꢋ.ꢊꢈ
ꢋ.ꢃꢌ
ꢋ.ꢅꢈ
−
ꢋ
μ, mm
θmax, deg
ꢄ.ꢇ
ꢋ.ꢄ
ꢋ.ꢆ
ꢃꢉ.ꢄꢄꢊ
ꢈꢊꢇꢌ
ꢃꢉ.ꢌꢌꢇ
ꢋꢊ ꢈꢉꢊ
ꢊꢉꢆꢊ
ꢃꢈ.ꢄꢄꢃ
ꢌꢈꢆꢌ
Refl. collected
Unique refl.
Rint
ꢊꢆꢃꢇ
ꢊꢈꢋꢉ
ꢄ.ꢄꢅꢈꢌ
ꢄ.ꢉꢇꢉ/ꢄ.ꢌꢊꢃ
ꢊꢋꢆꢊ
ꢄ.ꢄꢃꢇꢋ
ꢄ.ꢉꢄꢊ/ꢄ.ꢌꢋꢊ
ꢊꢆꢄꢆ
ꢄ.ꢄꢊꢋꢉ
ꢄ.ꢉꢋꢇ/ꢄ.ꢉꢌꢋ
ꢊꢄꢊꢆ
ꢄ.ꢄꢆꢈꢄ
–
ꢋꢉꢇꢅ
ꢋꢋꢉ
ꢄ.ꢄꢊꢆꢆ
ꢄ.ꢄꢇꢆꢈ
ꢋ.ꢄꢉꢈ
Min/max transm.
Refl. [F ꢁ>ꢁꢆ σ(F )]
ꢄ
ꢄ
Parameters
ꢋꢉꢇ
ꢋꢉꢇ
ꢃꢆꢊ
R [F ꢁ>ꢁꢆ σ(F )]
ꢄ.ꢄꢃꢇꢇ
ꢄ.ꢄꢉꢃꢇ
ꢋ.ꢄꢈꢃ
ꢄ.ꢄꢃꢅꢋ
ꢄ.ꢄꢈꢈꢄ
ꢋ.ꢄꢅꢉ
ꢄ.ꢄꢊꢄꢌ
ꢄ.ꢄꢉꢉꢌ
ꢋ.ꢄꢊꢄ
ꢋ
ꢄ
ꢄ
wRꢃ
GOF
Δρmax/min, e Å−ꢊ
ꢄ.ꢅꢋ/–ꢄ.ꢇꢊ
ꢄ.ꢃꢇ/–ꢄ.ꢃꢋ
ꢄ.ꢅꢋ/–ꢄ.ꢈꢆ
ꢄ.ꢊꢊ/–ꢄ.ꢊꢌ
complexes built up of Zn(NCS) melting is observed before
2
.4 Thermoanalytical investigations
2
the solid decomposes, but here we have no hint for such
To check if compounds 1-Cd and 1-Zn can be transformed a scenario from microscopic investigations [51, 52]. On
into 2-Cd, respectively 2-Zn or if different modifications further heating the sample mass decreases continuously
are obtained, these compound were investigated by dif- with no distinct mass steps, indicating the decomposi-
ferential thermoanalysis (DTA) and thermogravimetry tion of the compound formed as an intermediate after the
(
TG). Upon heating at 1°C/min both compounds show one first mass loss. To identify the intermediate compounds
mass loss at about 104°C for 1-Cd, respectively 51°C for the measurement was repeated, stopped after the first
-Zn, which is accompanied with endothermic events in mass loss, and the residue was investigated by XRPD.
the DTA curve (Fig. 6: top). The experimental mass loss of Comparison of the experimental XRPD pattern with those
1
3
2.8% for 1-Cd and of 33.8% for 1-Zn is in reasonable agree- calculated has shown, that 1-Cd and 1-Zn have been trans-
ment with that calculated for the removal of two 3-ethyl- formed into 2-Cd, respectively 2-Zn (Fig. 6: bottom).
pyridine ligands (Δmcalcd.ꢀ=ꢀ32.8% for 1-Cd and 35.2% for
1
-Zn). Therefore, one can assume that compounds with
the composition M(NCS) (3-ethylpyridine) (Mꢀ=ꢀCd, Zn) 2.5 Luminescence measurements
2
4
are formed as intermediates. The low decomposition tem-
perature also indicates that 1-Zn is not very stable at room As shown in Figure 7, the compounds 1-Cd, 2-Cd, 2-Zn and
temperature. It is noted, that the decomposition tempera- the 3-ethylpyridine ligand emit light in the blue spectral
tures are much lower than the boiling temperature of the range. Specifically, the emission spectrum of 1-Cd (Fig. 7,
−
1
3
-ethylpyridine ligand but it must be kept in mind, that dashed curve, νꢀ ꢀ=ꢀ25 707 cm ) consists of a broad band with
ex
−1
the experiment was performed under dynamic conditions. a maximum at 21 724 cm and full width at half maximum
−1
In this case the gaseous ligand, which is in equilibrium (FWHM) of 4748 cm , strongly overlapping with the emis-
−
1
with the solid, will be immediately removed leading to sion spectrum of 2-Cd (Fig. 7, gray curve, νꢀ ꢀ=ꢀ27 027 cm )
ex
−1
−1
further decomposition. However, for some other discrete with a maximum at 21 654 cm and FWHM of 4482 cm .
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20ꢀꢀꢀꢀꢁꢀT. Neumann et al.: New Cd(II) and Zn(II) thiocyanate coordination compounds
Fig. 6:ꢀDTA and TG curves of 1-Cd and 1-Zn measured at a heating
−
1
Fig. 7:ꢀTop: Emission spectra of the 3-ethylpyridine ligand (black
−1 −1
rate of 1°C min (top), and experimental XRPD patterns of the
residues obtained after the first TG step together with the patterns
calculated for 2-Cd and 2-Zn (bottom).
curve, νꢀ ꢁ=ꢁ 25 294 cm ), 1-Cd (dashed curve, νꢀ ꢁ=ꢁ25 707 cm ),
ex
ex
−
1
2
-Cd (gray curve, νꢀ ꢀ=ꢀ27 027 cm ), 2-Zn (pointed curve, νꢀ ꢁ=ꢁ
ex ex
−1
2
6 316 cm ). Bottom: Plot of color coordinates on the CIE (Commis-
sion internationale de l’éclairage) 1931 chromaticity diagram for the
3
-ethylpyridine ligand (♦), 1-Cd (■), 2-Cd (●) and 2-Zn ( ).
Similarly, the emission spectrum of 2-Zn (Fig. 7, pointed
−
1
curve, νꢀ ꢀ=ꢀ26 316 cm ) consists also of a broad band with a
ex
−
1
−1
−1
−1
maximum at 22 055 cm with FWHM of 5514 cm .
νꢀ ex ꢀ=ꢀ25 294 cm ), centered at 21 514 cm with FWHM
−1
The resemblance of these emission spectra for similar of 3106 cm . Consequently, the resultant CIE 1931 color
excitation energies is explained by the ligand-based nature coordinates present also close values for 1-Cd (xꢀ=ꢀ0.1879,
2+
2+
of the optical properties. Since Cd and Zn do not present yꢀ=ꢀ0.3254), 2-Cd (xꢀ=ꢀ0.1794, yꢀ=ꢀ0.2988), 2-Zn (xꢀ=ꢀ0.1802,
radiative electronic transitions, the optical properties of yꢀ=ꢀ0.2846) and 3-ethylpyridine (xꢀ=ꢀ0.1407, yꢀ=ꢀ0.2619).
compounds 1-Cd, 2-Cd and 2-Zn are based on the S →S
1
0
transition of the aromatic units within the organic ligand
2
+
2+
[
55], as is also observed for other Cd - and Zn -based com- 3 Conclusions
plexes, coordination polymers and metal-organic frame-
works [44–47, 50–52, 56–58]. This fact is confirmed by the In the present contribution, new Cd(NCS) and Zn(NCS)2
2
similarity of the emission spectra of 1-Cd, 2-Cd and 2-Zn to coordination compounds with 3-ethylpyridine as co-ligand
that of the 3-ethylpyridine ligand (Fig. 7, solid black curve, are reported. If an excess of 3-ethylpyridine is used in the
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T. Neumann et al.: New Cd(II) and Zn(II) thiocyanate coordination compoundsꢂꢁꢀꢀꢀꢂ121
synthesis, discrete octahedral complexes with only termi- 0.15 mmol) with 3-ethylpyridine (68.3 μL, 0.60 mmol) in
nally N-bonded anionic ligands are obtained. At lower metal 2.5 mL H O in a closed test tube at room temperature for
2
salt to 3-ethylpyridine ratio, compounds of composition 3 days. A crystalline powder was obtained by the reac-
M(NCS) (3-ethylpyridine) (Mꢀ=ꢀCd, Zn) are obtained, which tion of Cd(NCS) (57.2 mg, 0.25 mmol) with 3-ethylpyridine
2
2
2
in both cases can also be prepared by thermal decomposi- (1.0 mL, 8.9 mmol). Elemental analysis for C H CdN S
30
36
6 2
−1
tion of the 3-ethylpyridine rich precursor complexes. In the (657.188 g mol ): C 54.83, H 5.52, N 12.79, S 9.76; found C
Cd compound the cations are octahedrally coordinated and 54.12, H 5.38, N 12.63, S 9.80.
linked into linear chains by the anionic ligands, which is
a very common structure in this class of compounds. The
corresponding Zn compound shows a tetrahedral coordi- 4.1.3 Syntheses of Cd(NCS) (3-ethylpyridine) (2-Cd)
2
2
nation and therefore, this compound forms discrete com-
plexes with still terminally N-bonded thiocyanate anions. Single crystals suitable for X-ray structure determina-
The 3-ethylpyridine-rich compounds can be transformed tion were obtained by the reaction of Cd(NCS) (34.2 mg,
2
into the 3-ethylpyridine deficient compounds by anneal- 0.15 mmol) with 3-ethylpyridine (34.2 μL, 0.3 mmol)
ing, but the Zn compound 1-Zn already transforms into in 2.5 mL H O in a closed test tube for 3 days. A crystal-
2
2
-Zn on storage at room-temperature, because for Zn the line powder was obtained by the reaction of Cd(NCS)2
tetrahedral coordination is more stable. IR and Raman (114.3 mg, 0.50 mmol) with 3-ethylpyridine (112.3 μL,
spectroscopic investigations have shown, that it is difficult 1.00 mmol) in 1.5 mL H O. Elemental analysis for
for these compounds to distinguish the terminal and the C H CdN S (442.8777 g mol ): C 43.39, H 4.1, N 12.65, S
2
−1
1
6
18
4 2
bridging coordination modes of the anionic ligands. The 14.48; found C 41.80, H 3.83, N 12.34, S 14.69.
emission spectra of 1-Cd, 2-Cd, 2-Zn and the 3-ethylpyri-
dine ligand show broad bands in the blue spectral range
that originate from a ligand-based luminescence with no
significant differences between the different compounds.
4
.1.4 Synthesis of Zn(NCS) (3-ethylpyridine) (1-Zn)
2
4
Suitable single crystals for X-ray structure determina-
tion were obtained by the reaction of Zn(NCS) (45.4 mg,
2
0
.25 mmol) with 3-ethylpyridine (336.9 μL, 3.00 mmol)
4
Experimental section
in 1.5 mL H O in a closed test tube at room temperature
2
for five days. A phase pure polycrystalline powder was
obtained through stirring Zn(NCS) (0.5 mmol, 90.0 mg) in
4
.1 Synthesis
2
an excess of 3-ethylpyidine (17.8 mmol, 2.0 mL). It is noted
that this compound transforms into 2-Zn when stored at
ambient conditions.
4
.1.1 Syntheses of Cd(NCS) and Zn(NCS)
2
2
Ba(NCS) ꢀ·ꢀ3H O and 3-ethylpyridine were obtained from
2
2
Alfa Aesar and ZnSO ꢀ·ꢀH O, CdSO ꢀ·ꢀ8/3H O were obtained
4
2
4
2
4
.1.5 Synthesis of Zn(NCS) (3-ethylpyridine) (2-Zn)
2
2
from Merck. All chemicals were used without further
purification. Cd(NCS) and Zn(NCS) were prepared by
2
2
Suitable single crystals for X-ray structure determina-
tion were obtained by the reaction of Zn(NCS) (54.4 mg,
the reaction of equimolar amounts of ZnSO ꢀ·ꢀH O, respec-
4
2
2
tively CdSO ꢀ·ꢀ8/3H O and Ba(NCS) ꢀ·ꢀ3H O in water.
4
2
2
2
0
.30 mmol) with 3-ethylpyridine (34.2 μL, 0.30 mmol) in
The resulting white precipitates of BaSO were fil-
4
1
.0 mL ethanol in a closed test tube after 3 days. A crys-
talline powder was obtained by the reaction of Zn(NCS)2
90.8 mg, 0.50 mmol) with 3-ethylpyridine (112.3 μL,
.00 mmol) in 1.5 mL H O. Elemental analysis for
tered off and the filtrate was concentrated to complete
dryness resulting in white residues of Zn(NCS) and
2
(
Cd(NCS) . All crystalline powders were prepared by stir-
2
1
2
ring of the reactants in solution for 3 days at room tem-
perature. The residues were filtered off and washed with
water and dried in air.
−1
C H N S Zn (395.8577 g mol ): C 48.55, H 4.58, N 14.15, S
1
6
18
4 2
1
6.20; found C 48.004, H 4.61, N 13.78, S 16.10.
4
.1.2 Syntheses of Cd(NCS) (3-ethylpyridine) (1-Cd)
4.2 Elemental analysis
2
4
Single crystals suitable for X-ray structure determina- CHNS analyses were performed using an EURO EA elemen-
tion were obtained by the reaction of Cd(NCS) (34.3 mg, tal analyzer, fabricated by EURO VECTOR Instruments.
2
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1
22ꢀꢀꢀꢀꢁꢀT. Neumann et al.: New Cd(II) and Zn(II) thiocyanate coordination compounds
crystal data and details of the structure refinements can
be found in Table 2.
4
.3 Differential thermal analysis
and thermogravimetry (DTA-TG)
CCDC 1585623 (1-Cd), 1585625 (2-Cd), 1585626 (1-Zn)
DTA-TG measurements were performed in a dynamic nitro- and 1585624 (2-Zn) contain the supplementary crystallo-
gen atmosphere in Al O crucibles using a STA-PT1600 graphic data for this paper. These data can be obtained
2
3
thermobalance from Linseis. The instrument was cali- free charge from the Cambridge Crystallographic Data
brated using standard references materials. All measure- Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
−1
ments were performed with a flow rate of 75 mL min and
were corrected for buoyancy.
4
.6 IR and Raman spectroscopy
4
.4 X-ray powder diffraction (XRPD)
All IR data were obtained using an ATI Mattson Genesis
Series FTIR Spectrometer, control software: Wꢊꢋꢌꢊꢉꢍꢅ,
The measurements were performed using a stoe trans- from ATI Mattson. All Raman spectra were recorded using
mission powder diffraction system (STADI P) with CuKα1 a IFS 66/CS NIR Fourier-Transform Raman spectrometer
radiation (λꢀ=ꢀ1.540 598 Å) equipped with a MYTHEN 1K and a FRA 106 from Bruker.
detector from STOE & Cie.
4
.7 Luminescence measurements
4
.5 Single-crystal structure analyses
Luminescence spectra have been measured with a HORIBA
Data collections were performed with an imaging plate Jobin Yvon GmbH (Unterhaching, Germany) spectrometer
diffraction system IPDS-1 (1-Cd, 1-Zn, 2-Cd and 2-Zn) and (Fluorolog3) at room temperature, equipped with an iHR-
IPDS-2 (2-Zn-H O) from STOE & Cie using MoK radia- 320-FA Triple Grating spectrograph, R928P Photomulti-
2
α
tion. Structure solutions were performed with Sꢁꢂꢃꢄꢅ plier and a 450 W xenon lamp.
2
[59] and structure refinement was performed against F
using Sꢁꢂꢃꢄꢃ-2014 [60]. A numerical absorption correc- Acknowledgments: This project was supported by the
tion was applied using the programs X-Rꢂꢆ and X-Sꢁꢇꢈꢂ Deutsche Forschungsgemeinschaft (Project No. Na 720/5-2
of the program package X-Aꢉꢂꢇ [61–63]. All non-hydrogen and TE1147/1-1) and the State of Schleswig-Holstein. We
atoms except the disordered C and N atoms of lower occu- thank Prof. Dr. Wolfgang Bensch for access to his experi-
pancy in 2-Cd were refined with anisotropic displacement mental facilities.
parameters. The hydrogen atoms were positioned with
idealized geometry (methyl H atoms in 2-Zn allowed to
rotate but not to tip) and were refined isotropically with
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